1. Field of the Invention
The present invention relates to an electrostatic recording apparatus for carrying out an image recording by forming an electrostatic latent image on a dielectric recording medium and developing the formed electrostatic latent image, and more particularly, to an apparatus for generating ions in a solid ion recording head for forming the electrostatic latent image by using ion currents.
2. Description of the Background Art
As an ion recording head for forming an electrostatic latent image by using ion currents, one using a solid ion generator instead of a corona charger is known conventionally. Such a solid ion generator comprises an ion generation electrode and an induction electrode which are arranged on a dielectric substrate. In a solid ion recording head using such a solid ion generator, an acceleration electrode having ion outlet holes in correspondence with recording picture elements is placed in front of such a solid ion generator and a bias voltage as high as the electrostatic latent image contrast is applied to the solid ion generator in accordance with the recording signals, so as to control a flow of the ion currents for forming the electrostatic latent image on the dielectric recording medium.
In such a solid ion recording head using a solid ion generator, the high density ions can be generated and therefore the high speed recording faster than in a laser printer becomes possible, as described in detail in "The 4th international congress on advances in non-impact printing technologies", sponsored by SPSE, p. 394.
As an example of a conventional solid ion recording head, that disclosed in Japanese Patent Application Laid Open No. 54-78134 and U.S. Pat. No. 4,160,257 is shown in FIG. 1.
This solid ion recording head of FIG. 1 comprises an induction electrode 902 provided on one side of a dielectric substrate 901, and an ion generation electrode 903 provided on the other side of the dielectric substrate 901. The ion generation electrode 903 has a slit (or hole) 904 for concentrating the electric field such that the ions can be generated easily. When the alternating voltage 905 is applied between the induction electrode 902 and the ion generation electrode 903, a strong alternating electric field is generated in the slit 904 and the high density ions of positive and negative polarities are generated. Among the positive and negative ions so generated, only the ions of the positive polarity are selected out by a high bias voltage 906 of 1000 to 1600 V which is approximately equal to the electrostatic latent image voltage level applied to the ion generation electrode 903, and are subsequently transferred toward a dielectric recording medium 907. These ions transferring toward the dielectric recording medium 907 are then accelerated by a high acceleration voltage 909 of about 800 to 1200 V applied to an acceleration electrode 908 provided between the ion generation electrode 903 and the dielectric recording medium 907, and reach the dielectric recording medium 907 to form the electrostatic latent image according to the image signals. In this manner, the flow of the ion currents is controlled to be On and Off by using the bias voltage 906. The solid ion recording head has a number of recording head elements such as that shown in FIG. 1 arranged linearly in correspondence with a number of picture elements. Here, a corona charger used in a conventional electrophotography may be used instead of a solid ion generator.
However, such a conventional solid ion recording head has the following problems.
First, in the solid ion recording head, it is necessary to apply a voltage of 1000 to 1600 V which is as high as that of the electrostatic latent image voltage level on the dielectric recording medium 907 to the ion generation electrode 903 as a signal voltage in order to control the ion currents. More specifically, this is achieved by switching a switch 910 in accordance with the image signals and applying the bias voltage 906. As a result, in the electrostatic recording apparatus using such a solid ion recording head, it becomes necessary to use a driving IC of high withstand voltage. However, such a driving IC of high withstand voltage requires a large installation area such that it is not suitable for a high resolution head for which a high density installation is necessary. On the other hand, when the driving circuit is formed by using a driving IC of high withstand voltage and subdivided into matrix driven parts, it becomes difficult to carry out the gradation recording (multi-value recording) by using the pulse width control during the high speed recording and only the binary recording using On and Off control is possible.
Secondly, in the electrostatic recording apparatus using a conventional ion recording head, all the ions generated are transferred toward the dielectric recording medium 907. However, in this manner of recording, the amount of ion generation varies as the ion generation critical voltage changes depending on the surface state of the ion generation electrode 903, so that it has been difficult to form a uniform electrostatic latent image even in a case of a binary recording.
The Delfax Corporation of U.S.A. has developed a solid ion recording head in which the ion currents are On and Off controlled by switching the high frequency high voltage of about 3 KV.sub.p-p and 1 MHz to be applied to a solid ion generator for each picture element by using the signal voltages for each picture element, and the binary electrostatic latent image is formed on an insulative layer of the recording medium by using all the ions generated as the generated ions are accelerated by applying the high direct voltage of over 1 KV to a common acceleration electrode having ion outlet holes in correspondence with the picture elements. This solid ion recording head is capable of carrying out the high speed binary recording of up to 330 papers per minute for A4 size paper, and can be operated with only one maintenance operation for printing of a hundred thousand papers.
However, in general, the amount of ions generated by the solid ion generator is greatly affected by the environmental conditions, and because the above described solid ion recording head uses all the ions generated in forming the electrostatic latent image, so that there has been possibilities for the deterioration of the image quality as the amount of ions contributing to the electrostatic latent image varies depending on the environmental conditions. For this reason, the Delfax Corporation uses a crystalline mica for the dielectric substrate of the solid ion recording head because the crystalline mica remains stable for an extended period of time as it is not altered by the nitrate generated by the ion radiation and corona ion generation. This, however, gives rise to a problem that it is difficult to adapt this solid ion recording head to a mass production because of the difficulty in attaching the crystalline mica with a device substrate and forming electrodes on the crystalline mica by using a thick film printing technique.
Also, in such a solid ion recording head, it is necessary to have an accurate agreement between the size and the center of the ion generation hole of the solid ion generator and those of the ion outlet hole of the acceleration electrode for each picture element. When such an agreement is not achieved, the amount of ion generation can be varied, and the fluctuation in the amount of the ion generation determined by the accuracy of manufacturing technique can cause the concentration fluctuation on the recorded image.
Moreover, the solid ion recording head described above is capable of carrying out the high speed recording, but a special type of a driving circuit is necessary because the high frequency high voltage is used for each picture element, so that the size of the driving circuit becomes larger and it is difficult to form this driving circuit in a form of a driving IC.
There is a proposition for manufacturing the dielectric substrate with a material which can be adapted to a mass production by using the thick film printing technique, where the ion generation is stabilized by providing the dielectric substrate in a form of a double layer structure and heaters are used as the electrodes, and where the amount of ion generation can be appropriately controlled by adjusting the frequency of the alternating voltage. However, such a solid ion recording head is structurally equivalent to a capacitive load in which an amount of the alternating current increases when the frequency of the alternating voltage is increased. The power source of a high voltage, high frequency, and a large amount of current is quite expensive and can enlarge the size of the apparatus itself.
As a method of reducing the driving voltage for the ion recording head, there is a method disclosed in Japanese Patent Application Laid Open No. 61-255870 in which a control electric field is provided in a direction perpendicular to the ion current flow transported by a high speed air flow. By using this method, it becomes possible to reduce the driving voltage to be as low as about 30 V, as well as to carry out the multi-value recording, but a complicated electrode structure becomes necessary in order to provide the control electric field mentioned above, and therefore it is not suitable for the high density installation. Moreover, in this method, the speed of recording is determined by the speed of the air flow, and it is difficult to obtain a stable recording.
On the other hand, there is known a method in which a corona charger is used instead of the ion generator, the generated ion currents are pinched down by two control electrodes, and the flow of the ion currents is controlled by the signal voltages between two control electrodes. This method uses a relatively low control voltage of 120 V and is capable of obtaining a high contrast electrostatic latent image. In addition, a usual toner used in a general copy machine can be used for this method, and it is possible to carry out the analog gradation recording at the same quality as that can be obtained by a laser printer in which the gradation is achieved by the concentration of the picture elements, with the resolution lower than that of the laser printer.
However, there is a need to apply the high voltage for ion acceleration between the recording medium and the control electrodes so that it is necessary to bias the driving circuit by the high voltage.
Moreover, the amount of ions that can be generated by the corona charger is limited, so that the recording speed is accordingly limited to about 2 sheets/minute at best.
Furthermore, in this method, it is necessary to provide electrodes for pinching down the ion currents between the corona charger and the control electrodes, and there is a need for having an accurate agreement between the size and the center of the ion outlet hole of these electrodes and those of the ion outlet hole of the control electrodes. When such an agreement is not achieved, the amount of ion generation can be varied greatly, and the fluctuation in the amount of the ion generation determined by the accuracy of manufacturing technique can cause the concentration fluctuation on the recorded image.
In addition, this method uses the corona charger, so that it is difficult to solidify the ion recording head and therefore it is not suitable for the mass production.
Furthermore, the various conventional methods described so far have a problem that the ion recording head is polluted by the floating toner or the residual toner on the recording medium, such that the toner gets stuck in the ion outlet hole for the ion currents and obstructs the flow of the ion currents.